Aircraft design projects by Jenkinson and Marcman


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Aircraft design projects by Jenkinson and Marcman

  1. 1. Aircraft Design Projects “fm” — 2003/3/11 — page i — #1
  2. 2. Dedications To Jessica, Maria, Edward, Robert and Jonothan – in their hands rests the future. To my father, J. F. Marchman, Jr, for passing on to me his love of airplanes and to my teacher, Dr Jim Williams, whose example inspired me to pursue a career in education. “fm” — 2003/3/11 — page ii — #2
  3. 3. Aircraft Design Projects for engineering students Lloyd R. Jenkinson James F. Marchman III OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO “fm” — 2003/3/11 — page iii — #3
  4. 4. Butterworth-Heinemann An imprint of Elsevier Science Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Road, Burlington MA 01803 First published 2003 Copyright © 2003, Elsevier Science Ltd. All rights reserved No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publisher Permissions may be sought directly from Elsevier’s Science and Technology Rights Department in Oxford, UK: phone: (+44) (0) 1865 843830; fax: (+44) (0) 1865 853333; e-mail: You may also complete your request on-line via the Elsevier Science homepage (, by selecting ‘Customer Support’ and then ‘Obtaining Permissions’ British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0 7506 5772 3 Typeset by Newgen Imaging Systems (P) Ltd., India Printed in UK For information on all Butterworth-Heinemann publications visit our website at “fm” — 2003/3/11 — page iv — #4
  5. 5. Contents Preface Acknowledgements Introduction 1 Design methodology 2 xiii xvi xvii Preliminary design 2.1 2.2 2.3 2.4 2.5 2.6 2.7 1 Problem definition 2.1.1 The customers 2.1.2 Aircraft viability 2.1.3 Understanding the problem 2.1.4 Innovation 2.1.5 Organising the design process 2.1.6 Summary Information retrieval 2.2.1 Existing and competitive aircraft 2.2.2 Technical reports 2.2.3 Operational experience Aircraft requirements 2.3.1 Market and mission issues 2.3.2 Airworthiness and other standards 2.3.3 Environmental and social issues 2.3.4 Commercial and manufacturing considerations 2.3.5 Systems and equipment requirements Configuration options Initial baseline sizing 2.5.1 Initial mass (weight) estimation 2.5.2 Initial layout drawing Baseline evaluation 2.6.1 Mass statement 2.6.2 Aircraft balance 2.6.3 Aerodynamic analysis 2.6.4 Engine data 2.6.5 Aircraft performance 2.6.6 Initial technical report Refining the initial layout 2.7.1 Constraint analysis 2.7.2 Trade-off studies “fm” — 2003/3/10 — page v — #5 6 6 7 8 8 9 10 11 11 11 12 12 12 13 13 13 14 14 14 15 16 19 19 19 21 22 24 25 25 25 26 29
  6. 6. vi Contents 2.8 2.9 Refined baseline design Parametric and trade studies 2.9.1 Example aircraft used to illustrate trade-off and parametric studies 2.10 Final baseline configuration 2.10.1 Additional technical considerations 2.10.2 Broader-based considerations 2.11 Type specification 2.11.1 Report format 2.11.2 Illustrations, drawings and diagrams References 33 39 39 39 40 40 41 41 3 Introduction to the project studies 43 4 Project study: scheduled long-range business jet 46 47 49 50 50 50 51 51 52 53 54 54 55 56 57 57 58 59 60 61 62 62 63 63 67 68 70 70 75 76 78 79 80 82 82 85 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Introduction Project brief 4.2.1 Project requirements Project analysis 4.3.1 Payload/range 4.3.2 Passenger comfort 4.3.3 Field requirements 4.3.4 Technology assessments 4.3.5 Marketing 4.3.6 Alternative roles 4.3.7 Aircraft developments 4.3.8 Commercial analysis Information retrieval Design concepts 4.5.1 Conventional layout(s) 4.5.2 Braced wing/canard layout 4.5.3 Three-surface layout 4.5.4 Blended body layout 4.5.5 Configuration selection Initial sizing and layout 4.6.1 Mass estimation 4.6.2 Engine size and selection 4.6.3 Wing geometry 4.6.4 Fuselage geometry 4.6.5 Initial ‘baseline aircraft’ general arrangement drawing Initial estimates 4.7.1 Mass and balance analysis 4.7.2 Aerodynamic estimations 4.7.3 Initial performance estimates 4.7.4 Constraint analysis 4.7.5 Revised performance estimates 4.7.6 Cost estimations Trade-off studies 4.8.1 Alternative roles and layout 4.8.2 Payload/range studies “fm” — 2003/3/10 — page vi — #6 31 32
  7. 7. Contents 4.8.3 Field performance studies 4.8.4 Wing geometry studies 4.8.5 Economic analysis 4.9 Initial ‘type specification’ 4.9.1 General aircraft description 4.9.2 Aircraft geometry 4.9.3 Mass (weight) and performance statements 4.9.4 Economic and operational issues 4.10 Study review References 86 87 91 96 96 97 97 98 99 100 5 101 102 102 103 104 105 106 108 110 110 110 112 113 115 115 117 119 129 129 129 130 130 130 131 131 131 132 133 134 137 137 137 139 139 140 141 141 5.1 5.2 Project study: military training system Introduction Project brief 5.2.1 Aircraft requirements 5.2.2 Mission profiles 5.3 Problem definition 5.4 Information retrieval 5.4.1 Technical analysis 5.4.2 Aircraft configurations 5.4.3 Engine data 5.5 Design concepts 5.6 Initial sizing 5.6.1 Initial baseline layout 5.7 Initial estimates 5.7.1 Mass estimates 5.7.2 Aerodynamic estimates 5.7.3 Performance estimates 5.8 Constraint analysis 5.8.1 Take-off distance 5.8.2 Approach speed 5.8.3 Landing distance 5.8.4 Fundamental flight analysis 5.8.5 Combat turns at SL 5.8.6 Combat turn at 25 000 ft 5.8.7 Climb rate 5.8.8 Constraint diagram 5.9 Revised baseline layout 5.9.1 Wing fuel volume 5.10 Further work 5.11 Study review 5.11.1 Strengths 5.11.2 Weaknesses 5.11.3 Opportunities 5.11.4 Threats 5.11.5 Revised aircraft layout 5.12 Postscript References “fm” — 2003/3/10 — page vii — #7 vii
  8. 8. viii Contents 6 Project study: electric-powered racing aircraft 143 144 144 144 145 146 147 149 150 150 152 154 157 158 159 162 165 166 166 169 171 173 173 174 7 Project study: a dual-mode (road/air) vehicle 175 176 176 177 179 181 186 186 189 190 190 193 196 197 198 199 200 201 8 Project study: advanced deep interdiction aircraft 202 203 203 203 204 206 6.1 6.2 Introduction Project brief 6.2.1 The racecourse and procedures 6.2.2 History of Formula 1 racing 6.2.3 Comments from a racing pilot 6.2.4 Official Formula 1 rules 6.3 Problem definition 6.4 Information retrieval 6.4.1 Existing aircraft 6.4.2 Configurational analysis 6.4.3 Electrical propulsion system 6.5 Design concepts 6.6 Initial sizing 6.6.1 Initial mass estimations 6.6.2 Initial aerodynamic considerations 6.6.3 Propeller analysis 6.7 Initial performance estimation 6.7.1 Maximum level speed 6.7.2 Climb performance 6.7.3 Turn performance 6.7.4 Field performance 6.8 Study review References 7.1 7.2 7.3 7.4 7.5 7.6 Introduction Project brief (flying car or roadable aircraft?) Initial design considerations Design concepts and options Initial layout Initial estimates 7.6.1 Aerodynamic estimates 7.6.2 Powerplant selection 7.6.3 Weight and balance predictions 7.6.4 Flight performance estimates 7.6.5 Structural details 7.6.6 Stability, control and ‘roadability’ assessment 7.6.7 Systems 7.6.8 Vehicle cost assessment 7.7 Wind tunnel testing 7.8 Study review References 8.1 8.2 Introduction Project brief 8.2.1 Threat analysis 8.2.2 Stealth considerations 8.2.3 Aerodynamic efficiency “fm” — 2003/3/10 — page viii — #8
  9. 9. Contents 8.3 8.4 8.5 8.6 Problem definition Design concepts and selection Initial sizing and layout Initial estimates 8.6.1 Initial mass estimations 8.6.2 Initial aerodynamic estimations 8.7 Constraint analysis 8.7.1 Conclusion 8.8 Revised baseline layout 8.8.1 General arrangement 8.8.2 Mass evaluation 8.8.3 Aircraft balance 8.8.4 Aerodynamic analysis 8.8.5 Propulsion 8.9 Performance estimations 8.9.1 Manoeuvre performance 8.9.2 Mission analysis 8.9.3 Field performance 8.10 Cost estimations 8.11 Trade-off studies 8.12 Design review 8.12.1 Final baseline aircraft description 8.12.2 Future considerations 8.13 Study review References 9 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Project study: high-altitude, long-endurance (HALE) uninhabited aerial surveillance vehicle (UASV) Introduction Project brief 9.2.1 Aircraft requirements Problem definition Initial design considerations Information retrieval 9.5.1 Lockheed Martin U-2S 9.5.2 Grob Strato 2C 9.5.3 Northrop Grumman RQ-4A Global Hawk 9.5.4 Grob G520 Strato 1 9.5.5 Stemme S10VC Design concepts 9.6.1 Conventional layout 9.6.2 Joined wing layout 9.6.3 Flying wing layout 9.6.4 Braced wing layout 9.6.5 Configuration selection Initial sizing and layout 9.7.1 Aircraft mass estimation 9.7.2 Fuel volume assessment 9.7.3 Wing loading analysis 9.7.4 Aircraft speed considerations “fm” — 2003/3/10 — page ix — #9 208 210 213 215 216 217 221 227 228 228 233 233 234 241 242 242 250 254 259 261 263 263 267 268 268 270 271 271 272 272 275 275 276 276 277 277 277 278 279 280 280 281 282 283 283 285 285 286 ix
  10. 10. x Contents 9.7.5 Wing planform geometry 9.7.6 Engine sizing 9.7.7 Initial aircraft layout 9.7.8 Aircraft data summary 9.8 Initial estimates 9.8.1 Component mass estimations 9.8.2 Aircraft mass statement and balance 9.8.3 Aircraft drag estimations 9.8.4 Aircraft lift estimations 9.8.5 Aircraft propulsion 9.8.6 Aircraft performance estimations 9.9 Trade-off studies 9.10 Revised baseline layout 9.11 Aircraft specification 9.11.1 Aircraft description 9.11.2 Aircraft data 9.12 Study review References 288 290 292 293 294 294 297 298 299 300 300 305 305 307 307 307 308 309 10 Project study: a general aviation amphibian aircraft 310 311 311 312 312 312 313 313 314 314 316 317 318 318 318 318 321 323 323 324 325 325 328 329 11 Design organisation and presentation 331 332 332 332 333 335 10.1 10.2 Introduction Project brief 10.2.1 Aircraft requirements 10.3 Initial design considerations 10.4 Design concepts 10.5 Initial layout and sizing 10.5.1 Wing selection 10.5.2 Engine selection 10.5.3 Hull design 10.5.4 Sponson design 10.5.5 Other water operation considerations 10.5.6 Other design factors 10.6 Initial estimates 10.6.1 Aerodynamic estimates 10.6.2 Mass and balance 10.6.3 Performance estimations 10.6.4 Stability and control 10.6.5 Structural details 10.7 Baseline layout 10.8 Revised baseline layout 10.9 Further work 10.10 Study review References 11.1 11.2 Student’s checklist 11.1.1 Initial questions 11.1.2 Technical tasks Teamworking 11.2.1 Team development “fm” — 2003/3/10 — page x — #10
  11. 11. Contents 11.2.2 Team member responsibilities 11.2.3 Team leadership requirements 11.2.4 Team operating principles 11.2.5 Brainstorming 11.3 Managing design meetings 11.3.1 Prior to the meeting 11.3.2 Minutes of the meeting 11.3.3 Dispersed meetings 11.4 Writing technical reports 11.4.1 Planning the report 11.4.2 Organising the report 11.4.3 Writing the report 11.4.4 Referencing 11.4.5 Use of figures, tables and appendices 11.4.6 Group reports 11.4.7 Review of the report 11.5 Making a technical presentation 11.5.1 Planning the presentation 11.5.2 Organising the presentation 11.5.3 Use of equipment 11.5.4 Management of the presentation 11.5.5 Review of the presentation 11.6 Design course structure and student assessment 11.6.1 Course aims 11.6.2 Course objectives 11.6.3 Course structure 11.6.4 Assessment criteria 11.6.5 Peer review 11.7 Naming your aircraft Footnote 336 336 337 337 338 339 339 341 341 342 342 343 344 345 346 347 348 349 349 350 351 352 353 353 354 354 355 356 356 357 Appendix A: 359 360 360 Units and conversion factors Derived units Funny units Conversions (exact conversions can be found in British Standards BS350/2856) Some useful constants (standard values) Design data sources Technical books (in alphabetical order) Reference books Research papers Journals and articles The Internet 361 362 Appendix B: 363 363 365 365 366 366 Index 367 “fm” — 2003/3/10 — page xi — #11 xi
  12. 12. “fm” — 2003/3/10 — page xii — #12
  13. 13. Preface There are many excellent texts covering aircraft design from a variety of perspectives.1 Some of these are aimed at specific audiences ranging from practising aerospace engineers, to engineering students, to amateur airplane builders. Others cover specialized aspects of the subject such as undercarriage or propulsion system design. Some of these are quite detailed in their presentation of the design process while others are very general in scope. Some are overviews of all the basic aeronautical engineering subjects that come together in the creation of a design. University faculty that teach aircraft design courses often face difficult choices when evaluating texts or references for their students’ use. Many texts that are suitable for use in a design class are biased toward particular classes of aircraft such as military aircraft, general aviation, or airliners. A text that gives excellent coverage of design basics may prove slanted toward a class of aircraft different from that year’s project. Alternatively, those that emphasize the correct type of vehicle may treat design fundamentals in an unfamiliar manner. The situation may be further complicated in classes that have several teams of students working on different types of designs, some of which ‘fit’ the chosen text while others do not. Most teachers would prefer a text that emphasizes the basic thought processes of preliminary design. Such a text should encourage students to seek an understanding of the approaches and constraints appropriate to their design assignment before they venture too far into the analytical processes. On the other hand, students would like a text which simply tells them where to input their design objectives into a ‘black-box’ computer code or generalized spreadsheet, and preferably, where to catch the final design drawings and specifications as they are printed out. Faculty would like their students to begin the design process with a thorough review of their previous courses in aircraft performance, aerodynamics, structures, flight dynamics, propulsion, etc. Students prefer to start with an Internet search, hoping to find a solution to their problem that requires only minimal ‘tweaking’. The aim of this book is to present a two pronged approach to the design process. It is expected to appeal to both faculty and students. It sets out the basics of the design thought process and the pathway one must travel in order to reach an aircraft design goal for any category of aircraft. Then it presents a variety of design case studies. These are intended to offer examples of the way the design process may be applied to conceptual design problems typical of those actually used at the advanced level in academic and other training curricula. It does not offer a step-by-step ‘how to’ design guide, but shows how the basic aircraft preliminary design process can be successfully applied to a wide range of unique aircraft. In so doing, it shows that each set of design objectives presents its own peculiar collection of challenges and constraints. It also shows how the classical design process can be applied to any problem. “fm” — 2003/3/10 — page xiii — #13
  14. 14. xiv Preface Case studies provide both student and instructor with a valuable teaching/learning tool, allowing them to examine the way others have approached particular design challenges. In the 1970s, the American Institute of Aeronautics and Astronautics (AIAA) published an excellent series of design case studies2 taken from real aircraft project developments. These provided valuable insights into the development of several, then current, aircraft. Some other texts have employed case studies taken from industrial practice. Unfortunately, these tend to include aspects of design that are beyond the conceptual phase, and which are not covered in academic design courses. While these are useful in teaching design, they can be confusing to the student who may have difficulty discerning where the conceptual aspects of the design process ends and detailed design ensues. The case studies offered in this text are set in the preliminary design phase. They emphasize the thought processes and analyses appropriate at this stage of vehicle development. Many of the case studies presented in this text were drawn from student projects. Hence, they offer an insight into the conceptual design process from a student perspective. The case studies include design projects that won top awards in national and international design competitions. These were sponsored by the National Aeronautics and Space Administration (NASA), the US Federal Aviation Administration (FAA), and the American Institute of Aeronautics and Astronautics (AIAA). The authors bring a unique combination of perspectives and experience to this text. It reflects both British and American academic practices in teaching aircraft design to undergraduate students in aeronautical and aerospace engineering. Lloyd Jenkinson has taught aircraft design at both Loughborough University and Southampton University in the UK and Jim Marchman has taught both aircraft and spacecraft design at Virginia Tech in the US. They have worked together since 1997 in an experiment that combines students from Loughborough University and Virginia Tech in international aircraft design teams.3 In this venture, teams of students from both universities have worked jointly on a variety of aircraft design projects. They have used exchange visits, the Internet and teleconference communications to work together progressively, throughout the academic year, on the conceptual design of a novel aircraft. In this book, the authors have attempted to build on their experience in international student teaming. They present processes and techniques that reflect the best in British and American design education and which have been proven to work well in both academic systems. Dr Jenkinson also brings to this text his prior experience in the aerospace industry of the UK, having worked on the design of several successful British aircraft. Professor Marchman’s contribution to the text also reflects his experiences in working with students and faculty in Thailand and France in other international design team collaborative projects. The authors envision this text as supplementing the popular aircraft design textbooks, currently in use at universities around the world. Books such as those reviewed by Mason1 could be employed to present the detailed aspects of the preliminary design process. Working within established conceptual design methodology, this book will provide a clearer picture of the way those detailed analyses may be adapted to a wide range of aircraft types. It would have been impossible to write this book without the hard work and enthusiasm shown by many of our students over more years than we care to remember. Their continued interest in aircraft design project work and the smoothing of the difficulties they sometimes experienced in progressing through the work was our inspiration. We have also benefited from the many colleagues and friends who have been generous in sharing their encouragement and knowledge with us. Aircraft design educators seem “fm” — 2003/3/10 — page xiv — #14
  15. 15. Preface to be a special breed of engineers who selflessly give their effort and time to inspire anyone who wants to participate in their common interest. We are fortunate to count them as our friends. References 1 Bill Mason’s web page: 2 AIAA web page: 3 Jenkinson, L. R., Page, G. J., Marchman, J. F., ‘A model for international teaming in air- craft design education’, Journal of Aircraft Design, Vol. 3, No. 4, pp. 239–247, Elsevier, December 2000. “fm” — 2003/3/10 — page xv — #15 xv
  16. 16. Acknowledgements To all the students and staff at Loughborough and Southampton Universities who have, over many years, contributed directly and indirectly to my understanding of the design of aircraft, I would like to express my thanks and appreciation. For their help with proof reading and technical advice, I thank my friends Paul Eustace and Keith Payne. Our gratitude to all those people in industry who have provided assistance with the projects. Finally, to my wife and family for their support and understanding over the time when my attention was distracted by the writing of the book. Lloyd Jenkinson I would like to acknowledge the work done by the teams of Virginia Tech and Loughborough University aircraft design students in creating the designs which I attempted to describe in Chapters 7 and 10 and the contributions of colleagues such as Bill Mason, Nathan Kirschbaum, and Gary Page in helping guide those students in the design process. Without these people these chapters could not have been written. Jim Marchman “fm” — 2003/3/10 — page xvi — #16
  17. 17. Introduction It is tempting to title this book ‘Flights of Fancy’ as this captures the excitement and expectations at the start of a new design project. The main objective of this book is to try to convey this feeling to those who are starting to undertake aircraft conceptual design work for the first time. This often takes place in an educational or industrial training establishment. Too often, in academic studies, the curiosity and fascination of project work is lost under a morass of mathematics, computer programming, analytical methods, project management, time schedules and deadlines. This is a shame as there are very few occasions in your professional life that you will have the chance to let your imagination and creativity flow as freely as in these exercises. As students or young engineers, it is advisable to make the most of such opportunities. When university faculty or counsellors interview prospective students and ask why they want to enter the aeronautics profession, the majority will mention that they want to design aircraft or spacecraft. They often tell of having drawn pictures of aeroplanes since early childhood and they imagine themselves, immediately after graduation, producing drawings for the next generation of aircraft. During their first years in the university, these young men and women are often less than satisfied with their basic courses in science, mathematics, and engineering as they long to ‘design’ something. When they finally reach the all-important aircraft design course, for which they have yearned for so long, they are often surprised. They find that the process of design requires far more than sketching a pretty picture of their dream aircraft and entering the performance specifications into some all-purpose computer program which will print out a final design report. Design is a systematic process. It not only draws upon all of the student’s previous engineering instruction in structures, aerodynamics, propulsion, control and other subjects, but also, often for the first time, requires that these individual academic subjects be applied to a problem concurrently. Students find that the best aerodynamic solution is not equated to the best structural solution to a problem. Compromises must be made. They must deal with conflicting constraints imposed on their design by control requirements and propulsion needs. They may also have to deal with real world political, environmental, ethical, and human factors. In the end, they find they must also do practical things like making sure that their ideal wing will pass through the hangar door! An overview of the book This book seeks to guide the student through the preliminary stages of the aircraft design process. This is done by both explaining the process itself (Chapters 1 and 2) and by providing a variety of examples of actual student design projects (Chapters 3 “fm” — 2003/3/10 — page xvii — #17
  18. 18. xviii Introduction to 10). The projects have been used as coursework at universities in the UK and the US. It should be noted that the project studies presented are not meant to provide a ‘fill in the blank’ template to be used by future students working on similar design problems but to provide insight into the process itself. Each design problem, regardless of how similar it may appear to an earlier aircraft design, is unique and requires a thorough and systematic investigation. The project studies presented in this book merely serve as examples of how the design process has been followed in the past by other teams faced with the task of solving a unique problem in aircraft design. It is impossible to design aircraft without some knowledge of the fundamental theories that influence and control aircraft operations. It is not possible to include such information in this text but there are many excellent books available which are written to explain and present these theories. A bibliography containing some of these books and other sources of information has been added to the end of the book. To understand the detailed calculations that are described in the examples it will be necessary to use the data and theories in such books. Some design textbooks do contain brief examples on how the analytical methods are applied to specific aircraft. But such studies are mainly used to support and illustrate the theories and do not take an overall view of the preliminary design process. The initial part of the book explains the preliminary design process. Chapter 1 briefly describes the overall process by which an aircraft is designed. It sets the preliminary design stages into the context of the total transformation from the initial request for proposal to the aircraft first flight and beyond. Although this book only deals with the early stages of the design process, it is necessary for students to understand the subsequent stages so that decisions are taken wisely. For example, aircraft design is by its nature an iterative process. This means that estimates and assumptions have sometimes to be made with inadequate data. Such ‘guesstimates’ must be checked when more accurate data on the aircraft is available. Without this improvement to the fidelity of the analytical methods, subsequent design stages may be seriously jeopardized. Chapter 2 describes, in detail, the work done in the early (conceptual) design process. It provides a ‘route map’ to guide a student from the initial project brief to the validated ‘baseline’ aircraft layout. The early part of the chapter includes sections that deal with ‘defining and understanding the problem’, ‘collecting useful information’ and ‘setting the aircraft requirements’. This is followed by sections that show how the initial aircraft configuration is produced. Finally, there are sections illustrating how the initial aircraft layout can be refined using constraint analysis and trade-off studies. The chapter ends with a description of the ‘aircraft type specification’. This report is commonly used to collate all the available data about the aircraft. This is important as the full geometrical description and data will be needed in the detailed design process that follows. Chapter 3 introduces the seven project studies that follow (Chapters 4 to 10). It describes each of the studies and provides a format for the sequence of work to be followed in some of the studies. The design studies are not sequential, although the earlier ones are shown in slightly more detail. It is possible to read any of the studies separately, so a short description of each is presented. Chapters 4 to 10 inclusive contain each of the project studies. The projects are selected from different aeronautical applications (general aviation, civil transports, military aircraft) and range from small to heavy aircraft. For conciseness of presentation the detailed calculations done to support the final designs have not been included in these chapters but the essential input values are given so that students can perform their own analysis. The projects are mainly based on work done by students on aeronautical engineering degree courses. One of the studies is from industrial work and some have “fm” — 2003/3/10 — page xviii — #18
  19. 19. Introduction been undertaken for entry to design competitions. Each study has been selected to illustrate a different aspect of preliminary design and to illustrate the varied nature of aircraft conceptual design. The final chapter (11) offers guidance on student design work. It presents a set of questions to guide students in successfully completing an aircraft design project. It includes some observations about working in groups. Help is also given on the writing of technical reports and making technical presentations. Engineering units of measurement Experience in running design projects has shown that students become confused by the units used to define parameters in aeronautics. Some detailed definitions and conversions are contained in Appendix A at the end of the book and a quick résumé is given here. Many different systems of measurement are used throughout the world but two have become most common in aeronautical engineering. In the US the now inappropriately named ‘British’ system (foot, pound and second) is widely used. In the UK and over most of Europe, System International (SI) (metres, newton and second) units are standard. It is advised that students only work in one system. Confusion (and disaster) can occur if they are mixed. The results of the design analysis can be quoted in both types of unit by applying standard conversions. The conversions below are typical: 1 inch = 25.4 mm 1 sq. ft = 0.0929 sq. m 1 US gal = 3.785 litres 1 US gal = 0.833 Imp. gal 1 statute mile = 1.609 km 1 ft/s = 0.305 m/s 1 knot = 1.69 ft/s 1 pound force = 4.448 newtons 1 horsepower = 745.7 watts 1 foot = 0.305 metres 1 cu. ft = 28.32 litres 1 Imp. gal = 4.546 litres 1 litre = 0.001 cubic metres 1 nautical mile = 1.852 km 1 knot = 0.516 m/s 1 knot = 1.151 mph 1 pound mass = 0.454 kilogram 1 horsepower = 550 ft lb/s To avoid confusing pilots and air traffic control, some international standardization of units has had to be accepted. These include: Aircraft altitude – feet Aircraft range – nautical miles Aircraft forward speed – knots∗ Climb rate – feet per minute (∗ Be extra careful with the definition of units used for aircraft speed as pilots like to use airspeed in IAS (indicated airspeed as shown on their flight instruments) and engineers like TAS (true airspeed, the speed relative to the ambient air)). Fortunately throughout the world, the International Standard Atmosphere (ISA) has been adopted as the definition of atmospheric conditions. ISA charts and data can be found in most design textbooks. In this book, which is aimed at a worldwide readership, where possible both SI and ‘British’ units have been quoted. Our apologies if this confuses the text in places. “fm” — 2003/3/10 — page xix — #19 xix
  20. 20. xx Introduction English – our uncommon tongue Part of this book grew out of the authors’ collaboration in a program of international student design projects over several years. As we have reported our experiences from that program, observers have often noted that one thing that makes our international collaboration easier than some others is the common language. On the other hand, one thing we and our students have learned from this experience is that many of the aspects of our supposedly common tongue really do not have much in common. Pairing an Englishman and an American to create a textbook aimed at both the US, British and other markets is an interesting exercise in spelling and language skills. While (or is it whilst?) the primary language spoken in the United Kingdom and the United States grows from the same roots, it has very obviously evolved somewhat differently. An easy but interesting way to observe some of these differences is to take a page of text from a British book and run it through an American spelling check program. Checking an American text with an ‘English’ spell checker will produce similar surprises. We spell many words differently, usually in small ways. Is it ‘color’ or ‘colour’; do we ‘organize’ our work or ‘organise’ it? In addition, do we use double (“) or single (‘) strokes to indicate a quote or give emphasis to a word or phrase? Will we hold our next meeting at 9:00 am or at 9.00 am? (we won’t even mention the 24 hour clock!). There are also some obvious differences between terminology employed in the US and UK. Does our automobile have a ‘bonnet’ and a ‘boot’ or a ‘hood’ and a ‘trunk’ and does its engine run on ‘gasoline’ or ‘petrol’? American ‘airplanes’ have ‘landing gear’ while British ‘aeroplanes/airplanes or aircraft’ have ‘undercarriages’, does it have ‘reheat’ or an ‘afterburner’. Fortunately, most of us have watched enough television shows and movies from both countries to be comfortable with these differences. As we have pieced together this work we have often found ourselves (and our computer spell checkers) editing each other’s work to make it conform to the conventions in spelling, punctuation, and phraseology, assumed to be common to each of our versions of this common language. The reader may find this evident as he or she goes from one section of the text to another and detects changes in wording and terminology which reflect the differing conventions in language use in the US and UK. It is hoped that these variations, sometimes subtle and sometimes obvious, will not prove an obstacle to the reader’s understanding of our work but will instead make it more interesting. Finally All aircraft projects are unique, therefore, it is impossible to provide a ‘template’ for the work involved in the preliminary design process. However, with knowledge of the detail steps in the preliminary design process and with examples of similar project work, it is hoped that students will feel freer to concentrate on the innovative and analytical aspects of the project. In this way they will develop their technical and communication abilities in the absorbing context of preliminary aircraft design. “fm” — 2003/3/10 — page xx — #20
  21. 21. 1 Design methodology The start of the design process requires the recognition of a ‘need’. This normally comes from a ‘project brief’ or a ‘request for proposals (RFP)’. Such documents may come from various sources: • Established or potential customers. • Government defence agencies. • Analysis of the market and the corresponding trends from aircraft demand. • Development of an existing product (e.g. aircraft stretch or engine change). • Exploitation of new technologies and other innovations from research and development. It is essential to understand at the start of the study where the project originated and to recognise what external factors are influential to the design before the design process is started. At the end of the design process, the design team will have fully specified their design configuration and released all the drawings to the manufacturers. In reality, the design process never ends as the designers have responsibility for the aircraft throughout its operational life. This entails the issue of modifications that are found essential during service and any repairs and maintenance instructions that are necessary to keep the aircraft in an airworthy condition. The design method to be followed from the start of the project to the nominal end can be considered to fall into three main phases. These phases are illustrated in Figure 1.1. The preliminary phase (sometimes called the conceptual design stage) starts with the project brief and ends when the designers have found and refined a feasible baseline design layout. In some industrial organisations, this phase is referred to as the ‘feasibility study’. At the end of the preliminary design phase, a document is produced which contains a summary of the technical and geometric details known about the baseline design. This forms the initial draft of a document that will be subsequently revised to contain a thorough description of the aircraft. This is known as the aircraft ‘Type Specification’. The next phase (project design) takes the aircraft configuration defined towards the end of the preliminary design phase and involves conducting detailed analysis to improve the technical confidence in the design. Wind tunnel tests and computational fluid dynamic analysis are used to refine the aerodynamic shape of the aircraft. Finite element analysis is used to understand the structural integrity. Stability and control analysis and simulations will be used to appreciate the flying characteristics. Mass and balance estimations will be performed in increasingly fine detail. Operational factors (cost, maintenance and marketing) and manufacturing processes will be investigated “chap01” — 2003/3/10 — page 1 — #1
  22. 22. 2 Aircraft Design Projects Preliminary design Costs and effort Build-up Project design Detail design Manufacturing Testing Timescale Fig. 1.1 The design process to determine what effects these may have on the final design layout. All these investigations will be done so that the company will be able to take a decision to ‘proceed to manufacture’. To do this requires knowledge that the aircraft and its novel features will perform as expected and will be capable of being manufactured in the timescales envisaged. The project design phase ends when either this decision has been taken or when the project is cancelled. The third phase of the design process (detail design) starts when a decision to build the aircraft has been taken. In this phase, all the details of the aircraft are translated into drawings, manufacturing instructions and supply requests (subcontractor agreements and purchase orders). Progressively, throughout this phase, these instructions are released to the manufacturers. Clearly, as the design progresses from the early stages of preliminary design to the detail and manufacturing phases the number of people working on the project increases rapidly. In a large company only a handful of people (perhaps as few as 20) will be involved at the start of the project but towards the end of the manufacturing phase several thousand people may be employed. With this build-up of effort, the expenditure on the project also escalates as indicated by the curved arrow on Figure 1.1. Some researchers1 have demonstrated graphically the interaction between the cost expended on the project, the knowledge acquired about the design and the resulting reduction in the design freedom as the project matures. Figure 1.2 shows a typical distribution. These researchers have argued for a more analytical understanding of the requirement definition phase. They argue that this results in an increased understanding of the effects of design requirements on the overall design process. This is shown on Figure 1.2 as process II, compared to the conventional methods, process I. Understanding these issues will increase design flexibility, albeit at a slight increase in initial expenditure. Such analytical processes are particularly significant in military, multirole, and international projects. In such case, fixing requirements too firmly and too early, when little is known about the effects of such constraints, may have considerable cost implications. Much of the early work on the project is involved with the guarantee of technical competence and efficiency of the design. This ensures that late changes to the design “chap01” — 2003/3/10 — page 2 — #2
  23. 23. Design methodology % 100 Process II Co Process I st p ex en de d Cost Design flexibility II I 0 A B C D Region Task A B C D Timescale Defining requirements Conceptual design phase Project design phase Detail design phase Fig. 1.2 Design flexibility layout are avoided or, at best, reduced. Such changes are expensive and may delay the completion of the project. Managers are eager to validate the design to a high degree of confidence during the preliminary and project phases. A natural consequence of this policy is the progressive ‘freezing’ of the design configuration as the project matures. In the early preliminary design stages any changes can (and are encouraged to) be considered, yet towards the end of the project design phase only minor geometrical and system modifications will be allowed. If the aircraft is not ‘good’ (well engineered) by this stage then the project and possibly the whole company will be in difficulty. Within the context described above, the preliminary design phase presents a significant undertaking in the success of the project and ultimately of the company. Design project work, as taught at most universities, concentrates on the preliminary phase of the design process. The project brief, or request for proposal, is often used to define the design problem. Alternatively, the problem may originate as a design topic in a student competition sponsored by industry, a government agency, or a technical society. Or the design project may be proposed locally by a professor or a team of students. Such design project assignments range from highly detailed lists of design objectives and performance requirements to rather vague calls for a ‘new and better’ replacement for existing aircraft. In some cases student teams may even be asked to develop their own design objectives under the guidance of their design professor. To better reflect the design atmosphere in an industry environment, design classes at most universities involve teams of students rather than individuals. The use of multidisciplinary design teams employing students from different engineering disciplines is being encouraged by industry and accreditation agencies. The preliminary design process presented in this text is appropriate to both the individual and the team design approach although most of the cases presented in later chapters involved teams of design students. While, at first thought, it may appear that the team approach to design will reduce the individual workload, this may not be so. “chap01” — 2003/3/10 — page 3 — #3 3
  24. 24. 4 Aircraft Design Projects The interpersonal dynamics of working in a team requires extra effort. However, this greatly enhances the design experience and adds team communications, management and interpersonnel interaction to the technical knowledge gained from the project work. It is normal in team design projects to have all students conduct individual initial assessments of the design requirements, study comparable aircraft, make initial estimates for the size of their aircraft and produce an initial concept sketch. The full team will then begin its task by examining these individual concepts and assessing their merits as part of their team concept selection process. This will parallel the development of a team management plan and project timeline. At this time, the group will allocate various portions of the conceptual design process to individuals or small groups on the team. At this point in this chapter, a word needs to be said about the role of the computer in the design process. It is natural that students, whose everyday lives are filled with computer usage for everything from interpersonal communication to the solution of complex engineering problems, should believe that the aircraft design process is one in which they need only to enter the operational requirements into some supercomputer and wait for the final design report to come out of the printer (Figure 1.3). Indeed, there are many computer software packages available that claim to be ‘aircraft design programs’ of one sort or another. It is not surprising that students, who have read about new aircraft being ‘designed entirely on the computer’ in industry, believe that they will be doing the same. They object to wasting time conducting all of the basic analyses and studies recommended in this text, and feel that their time would be much better spent searching for a student version of an all-encompassing aircraft design code. They believe that this must be available from Airbus or Boeing if only they can find the right person or web address. While both simple aircraft ‘design’ codes and massive aerospace industry CAD programs do exist and do play important roles, they have not yet replaced the basic processes outlined in this text. Simple software packages which are often available freely at various locations on the Internet, or with many modern aeronautical engineering texts, can be useful in the specialist design tasks if one understands the assumptions and limitations implicit in their analysis. Many of these are simple computer codes based on Design your own airplane in 5 min Output Fig. 1.3 Student view of design “chap01” — 2003/3/10 — page 4 — #4
  25. 25. A/C F PER STRUCTURES OAER IC AM DYN AB ST & PRO PU LSI ON Design methodology LU NT CO 2 AM F Fig. 1.4 The ‘real’ design process the elementary relationships used for aircraft performance, aerodynamics, and stability and control calculations. These have often been coupled to many simplifying assumptions for certain categories of aircraft (often home-built general aviation vehicles). The solutions which can be obtained from many such codes can be obtained more quickly, and certainly with a much better understanding of the underlying assumptions, by using directly the well-known relationships on which they are based. In our experience, if students spent half the time they waste searching for a design code (which they expect will provide an instant answer) on thinking and working through the fundamental relationships with which they are already supposedly familiar, they would find themselves much further along in the design process. The vast and complex design computer programs used in the aerospace industry have not been created to do preliminary work. They are used to streamline the detail design part of the process. Such programs are not designed to take the initial project requirements and produce a final design. They are used to take the preliminary design, which has followed the step-by-step processes outlined in this text, and turn it into the thousands of detailed CAD drawings needed to develop and manufacture the finished vehicle. It is the task of the aircraft design students to learn the processes which will take them from first principles and concepts, through the conceptual and preliminary design stages, to the point where they can begin to apply detailed design codes (Figure 1.4). At this point in time, it is impossible to envisage how the early part of the design process will ever be replaced by off-the-shelf computer software that will automatically design novel aircraft concepts. Even if this program were available, it is probably not a substitute for working steadily through the design process to gain a fundamental understanding of the intricacies involved in real aircraft design. Reference 1 Mavris, D. et al., ‘Methodology for examining the simultaneous impact of requirements, vehicle characteristics and technologies on military aircraft design’, ICAS 2000, Harrogate UK, August 2000. “chap01” — 2003/3/10 — page 5 — #5 5
  26. 26. 2 Preliminary design Conceptual design is the organised application of innovation to a real problem to produce a viable product for the customer. (Anon.) As previously described, the preliminary design phase starts with the recognition of need. It continues until a satisfactory starting point for the conceptual design phase has been identified. The aircraft layout at the end of the phase is referred to as the ‘baseline’ configuration. Between these two milestones there are a number of distinctive, and partially sequential, stages to be investigated. These stages are shown in Figure 2.1 and described below: 2.1 Problem definition For novice aircraft designers the natural tendency when starting a project is to want to design aircraft. This must be resisted because when most problems are originally presented they do not include all the significant aspects surrounding the problem. As a lot of time and effort will be spent on the design of the aircraft, it is important that all the criteria, constraints and other factors are recognised before starting, otherwise a lot of work and effort may be wasted. For this reason, the first part of the conceptual design phase is devoted to a thorough understanding of the problem. The definition of conceptual design quoted above raises a number of questions that are useful in analysing the problem. For example (in reverse order to the above definition): 1. Who are the customers? 2. How should we assess if the product is viable? 3. Can we completely define the problem in terms that will be useful to the technical design process? 4. What are the new/novel features that we hope to exploit to make our design better than the existing competition and to build in flexibility to cater for future developments? 5. What is the best way to tackle the problem and how will this be managed? These questions are used to gain more insight into the definition of the problem as explained below. “chap02” — 2003/3/10 — page 6 — #1
  27. 27. Preliminary design Problem definition Project brief Information retrieval Aircraft requirements Configuration options Initial sizing Baseline evaluation Constraint analysis Trade studies Refined baseline Parametric analysis Final baseline design Baseline analysis Aircraft type specification Fig. 2.1 The preliminary design flowchart 2.1.1 The customers Who are your ‘customers’? They are not only the purchasers of the aircraft; many groups of people and organisations will have an interest in the design and their expectations and opinions should be determined. For example, it would be technically straightforward to design a new supersonic airliner to replace Concorde. The operating and technical issues are now well understood. However, the environmental lobby (who want to protect the upper atmosphere from further contamination) and the airport noise abatement groups have such political influence as to render the project unfeasible at this time. For all new designs it is necessary to identify all the influential people and find out their views before starting the project. Who are the influential people? • Obviously at the top of the list are the clients (the eventual purchasers of the aircraft). • Their customers (people who fly and use the aircraft, people who operate and maintain it, etc.). • Your technical director, departmental head and line supervisor (these have a responsibility for the company and its shareholders to make a reasonable return on investments). • Your sales team (they know the market and understand customers and they will eventually have to market the aircraft). “chap02” — 2003/3/10 — page 7 — #2 7
  28. 28. 8 Aircraft Design Projects • As a student, your academic supervisors and examiners (what is it that they expect to see from the project work). It is useful to make a list of those people who you think will be important to the project and then find out what views they have. In academic courses the available timescale and facility to accomplish this consultation fully may not be available. In this case, set up your own focus groups and role-play to try to appreciate the expected opinions of various groups. 2.1.2 Aircraft viability It will be impossible to make rational decisions during the detailed design stages unless you can clearly establish how the product/aircraft is to be judged. Often this is easier said than done, as people will have various views on what are the important criteria (i.e. what you should use to make judgements). The aircraft manufacturing company and particularly its directors will want the best return on their investments (ROI). Unfortunately, so many non-technical issues are associated with ROI that it is too complicated to be used as a design criterion in the initial stages of the project. In the early days aircraft designers solved this dilemma by adopting aircraft mass (weight) as their minimising criteria. They knew that aircraft mass directly affected most of the performance and cost aspects and it had the advantage of being easy to estimate and control. Without any other information about design criteria, minimum mass is still a valid overall criterion to use. As more knowledge about the design and its operating regime becomes available it is possible to use a more appropriate parameter. For example, minimum direct operating cost (DOC) is frequently used for civil transport aircraft. For military aircraft, total life cycle cost (LCC), operational effectiveness (e.g. lethality, survivability, dependability, etc.) are more appropriate. High performance aircraft may be assessed by their operating parameters (e.g. maximum speed, turn rate, sink rate). Some time ago A. W. Bishop of British Aerospace observed: The message is clear – if everyone can agree beforehand on how to measure the effectiveness of the design, then the designer has a much simpler task. But even if everyone does not agree, the designer should still quantify his own ideas to give himself a sensible guide. The procedure is therefore relatively simple – ask all those groups and individuals, who you feel are important to the project, how they would assess project effectiveness. Add any weightings you feel are appropriate to these opinions and decide for yourself what criteria should be adopted (or get the project group to decide if you are not working alone). Remember that the criteria must be capable of being quantified and related to the design parameters. Criteria such as ‘quality’, ‘goodness’ and ‘general effectiveness’ are of no use unless such a description can be translated into meaningful design parameters. For example, the effectiveness of a fighter aircraft may be judged by its ability to manoeuvre and launch missiles quicker than an opponent. 2.1.3 Understanding the problem It is unusual if the full extent of the problem is included in the initial project brief. Often the subtlety of the problem is not made clear because the people who draft the problem are too familiar with the situation and incorrectly assume that the design team will be equally knowledgeable. It is also found that the best solution to a problem is always “chap02” — 2003/3/10 — page 8 — #3
  29. 29. Preliminary design found by considering the circumstances surrounding the problem in as broad a manner as possible. This procedure has been called ‘system engineering’. In this approach, the aircraft is considered only as one component in the total operating environment. The design of the aircraft is affected by the design of all the components in the whole system. For example, a military training aircraft is only one element in the airforce flight/pilot training process. There are many other parts to such a system including other aircraft, flight simulators and ground schools. The training aircraft is also part of the full operational activity of the airforce and cannot be divorced from other aircraft in the service, the maintenance/service sector, the flight operations and other airport management activities. On the other hand, the training aircraft itself can be considered as a total system including airframe, flight control, engine management, weapon on sensor systems, etc. All of these systems will interact to influence the total design of the aircraft. Such considerations may lead to conflicts in the realisation of the project. For example, although the airforce may have a particular view of the aircraft, the manufacturers may have a different perspective. The airforce will only be focused on their aircraft but the manufacturers will want the aircraft to form part of a family of aircraft, which will have commercial opportunities beyond the supply to the national airforce. Within this context the aircraft may not be directly optimised for a particular role. The best overall configuration for the aircraft will be a compromise between, sometimes competing, requirements. It is the designer’s responsibility to consider the layout from all the different viewpoints and to make a choice on the preferred design. He therefore needs to understand all aspects of the overall system in which the aircraft will operate. Some of the most notable past failures in aircraft projects have arisen due to designs initially being specified too narrowly. Conversely, successful designs have been shown to have considerable flexibility in their design philosophy. Part of the problem definition task is to identify the various constraints to which the aircraft must conform. Such constraints will arise from performance and operational requirements, airworthiness requirements, manufacturing considerations, and limitation on resources. There will also be several non-technical constraints that must be recognised. These may be related to political, social, legal, economic, and commercial issues. However, it is important that the problem is not overconstrained as this may lead to no feasible solution existing. To guard against this it is necessary to be forceful in only accepting constraints that have been fully justified and their consequences understood. For technical constraints (e.g. field performance, climb rate, turn performance, etc.) there will be an opportunity to assess their influences on the design in the later stages (a process referred to as constraint analysis). Non-technical restrictions are more difficult to quantify and therefore must be examined carefully. In general, the problem definition task can be related to the following questions: • Has the problem been considered as broadly as possible? (i.e. have you taken a systems approach?) • Have you identified all the ‘real’ constraints to the solution of the problem? • Are all the constraints reasonable? • Have you thoroughly examined all the non-technical constraints to determine their suitability? (Remember that such constraints will remain unchallenged after this time.) 2.1.4 Innovation The design and development of a new aircraft is an expensive business. The people who invest in such an enterprise need to be confident that they will get a safe and profitable “chap02” — 2003/3/10 — page 9 — #4 9
  30. 30. 10 Aircraft Design Projects return on their outlay. The basis for confidence in such projects lies in the introduction and exploitation of new technologies and other innovations. Such developments should give an operational and commercial advantage to the new design to make it competitive against existing and older products. Innovation is therefore an essential element in new aircraft design. The downside of introducing new technology is the increase in commercial risk. The balancing of risk against technical advantage is a fundamental challenge that must be accepted by the designers. Reduction of technological risk will be a high priority within the total design process. Empirical tests and analytical verification of the effects of innovative features are the designer’s insurance policy. Innovation does not just apply to the introduction of new technology. Novel business and commercial arrangements and new operational practices may be used to provide a commercial edge to the new design. Whatever is planned, the designer must be able to identify it early so that he can adjust the baseline design accordingly. The designers should be able to answer the following questions: • What are the new technologies and other innovations that will be incorporated into the design? • How will such features provide an advantage over existing/competing aircraft? • If the success of the innovation is uncertain, how can the risk to the project be mitigated? 2.1.5 Organising the design process Gone are the days, if they ever existed, of a project being undertaken by an individual working alone in a back room. Modern design practice is the synthesis of many different skills and expertise. Such combination of talent, as in an orchestra, requires organisation and management to ensure that all players are using the same source of information. The establishment of modern computer assisted design (CAD) software and other information technology (IT) developments allows disparate groups of specialists and managers to be working on the same design data (referred to in industry as ‘concurrent engineering’). The organisation of such systems demands careful planning and management. Design-build teams are sometimes created to take control of specific aircraft types within a multi-product company. The design engineer is central to such activity and therefore a key team player. It is essential for him to know the nature of the team structure, the design methods to be adopted, the standards to be used, the facilities to be required, and not least, the work schedules and deadlines to be met. Such considerations are particularly significant in student project work, as there are many other demands on team members. All students will have to personally time-manage all their commitments. Whether the team is selected by an advising faculty member or is self-selected, students will face numerous challenges during the course of a design project. In most student design projects the organisation of the work is managed by the ‘design team’. Good team organisation and an agreed management structure are both essential to success. These issues are discussed in detail in Chapter 11, with particular emphasis to teaming issues in sections 11.2 and 11.3 respectively. When working in a team environment, students are advised to consult these sections before attempting to proceed with the preliminary design. “chap02” — 2003/3/10 — page 10 — #5
  31. 31. Preliminary design 2.1.6 Summary The descriptions above indicate that there is a lot of work and effort to be exerted before it is possible to begin the laying-out of the aircraft shape. Each project is different so it is impossible to produce a template to use for the design process. The only common factor is that if you start the design without a full knowledge of the problem then you will, at best, be wasting your time but possibly also making a fool of yourself. Use the comments and questions above to gain a complete understanding of the problem. Write out a full description of the problem in a report to guide you in your subsequent work. An excellent way for design teams to begin this process of understanding the design problem is the use of the process known as ‘brainstorming’. This is discussed in more detail in section 11.2.5. Brainstorming is essentially a process in which all members of a team are able to bring all their ideas about the project to the table with the assurance that their ideas, no matter how far-fetched they may at first appear, are considered by the team. Without such an open mind, a team rarely is able to gain a complete understanding of the problem. 2.2 Information retrieval Later stages of the design process will benefit from knowledge of existing work published in the area of the project. Searching for such information will involve three tasks: 1. Finding data on existing and competitive aircraft. 2. Finding technical reports and articles relating to the project area and any advanced technologies to be incorporated. 3. Gathering operational experience. 2.2.1 Existing and competitive aircraft The first of these searches is relatively straightforward to accomplish. There are several books and published surveys of aircraft that can be easily referenced. The first task is to compile a list of all the aircraft that are associated with the operational area. For example, if we are asked to design a new military trainer we would find out what training aircraft are used by the major air forces in the world. This is published in the reviews of military aircraft, in magazines like Flight International and Aviation Week. Systematically go through this list, progressively gathering information and data on each aircraft. A spreadsheet is the best way of recording numerical values for common parameters (e.g. wing area, installed thrust, aircraft weights (or masses), etc.). A database is a good way to record other textural data on the aircraft (e.g. when first designed and flown, how many sold and to whom, etc.). The geometrical and technical data can be used to obtain derived parameters (e.g. wing loading, thrust to weight ratio, empty weight fraction, etc.). Such data will be used to assist subsequent technical design work. It is possible, using the graph plotting facilities of modern spreadsheet programs, to plot such parameters for use in the initial sizing of the aircraft. For instance, a graph showing wing loading against thrust loading for all your aircraft will be useful in selecting specimen aircraft to be used in comparison with your design. Such a plot also allows “chap02” — 2003/3/10 — page 11 — #6 11
  32. 32. 12 Aircraft Design Projects operational differences between different aircraft types to be identified. Categories of various aircraft types can be identified. 2.2.2 Technical reports As there are so many technical publications available, finding associated technical reports and articles can be time consuming. A good search engine on a computerbased information retrieval system is invaluable in this respect. Unfortunately, such help is not always available but even when it is, the database may not contain recent articles. Older, but still quite relevant, technical articles might also be easily missed when a search relies on computer search and retrieval systems. All computer search systems are very dependent on the user’s ability to choose key words which will match those used by whoever catalogued the material in the search system database. Success with such systems is often both difficult and incomplete as the user and the computer try to match an often quite different set of key words to describe a common subject. It becomes somewhat of a game, in which two people with different backgrounds try to describe the same physical object based on their own experiences. Often, a manual search of shelves in a library will product far better results in less time. Manual search is more laborious but such effort is greatly rewarded when appropriate material is found. This makes subsequent design work easier and it provides extra confidence to the final design proposal. An excellent place to start a technical search is with the reference section at the end of each chapter in your preferred textbooks. Start with a text with which you are already familiar and track down relevant references. Do this either by using computer methods, or in a manual search of the library shelves. This can rapidly lead to an expanding array of background material as subsequent reference lists, in the newly found reports (etc.), are also interrogated. 2.2.3 Operational experience One of the best sources of information, data and advice comes from the existing area of operation appropriate to your project. People and organisations that are currently involved with your study area are often very willing to share their experiences. However, treat such opinions with due caution as individual responses are sometimes not representative of the overall situation. The best advice on information retrieved is to collect as much as you can in the time available and to keep your lines of enquiry open so that new information can be considered as it becomes available throughout the design process. 2.3 Aircraft requirements From the project brief and the first two stages of the design process it is now possible to compile a statement regarding the requirements that the aircraft should meet. Such requirements can be considered under five headings: 1. Market/Mission 2. Airworthiness/other standards 3. Environment/Social “chap02” — 2003/3/10 — page 12 — #7
  33. 33. Preliminary design 4. Commercial/Manufacturing 5. Systems and equipment The detail to be considered under each of these headings will naturally vary depending on the type of aircraft. Some general advice for each section is offered below but it will also be necessary to consider specific issues relating to your design. 2.3.1 Market and mission issues The requirements associated with the mission will generally be included in the original project brief. Such requirements may be in the form of point performance values (e.g. field length, turn rates, etc.), as a description of the mission profile(s), or as operational issues (e.g. payload, equipment to be carried, offensive threats, etc.). The market analysis that was undertaken in the problem definition phase might have produced requirements that are associated with commonality of equipment or engines, aircraft stretch capability, multi-tasking, costs and timescales. 2.3.2 Airworthiness and other standards For all aircraft designs, it is essential to know the airworthiness regulations that are appropriate. Each country applies its own regulations for the control of the design, manufacture, maintenance and operation of aircraft. This is done to safeguard its population from aircraft accidents. Many of these national regulations are similar to the European Joint Airworthiness Authority (JAA) and US-Federal Aviation Administration (FAA) rules.1,2 Each of these regulations contains specific operational requirements that must be adhered to if the aircraft is to be accepted by the technical authority (ultimately the national government from which the aircraft will operate). Airworthiness regulations always contain conditions that affect the design of the aircraft (e.g. for civil aircraft the minimum second segment climb gradient at take-off with one engine failed). Although airworthiness documents are not easy to read because they are legalistic in form, it is important that the design team understands all the implications relating to their design. Separate regulations apply to military and civil aircraft types and to different classes of aircraft (e.g. very light aircraft, gliders, heavy aircraft, etc.). It is also important to know what operational requirements apply to the aircraft (e.g. minimum number of flight crew, maintenance, servicing, reliability, etc.). The purchasers of the aircraft may also insist that particular performance guarantees are included in the sales contract (e.g. availability, timescale, fuel use, etc.). Obviously all the legal requirements are mandatory and must be met by the aircraft design. The design team must therefore be fully conversant with all such conditions. 2.3.3 Environmental and social issues Social implications on the design and operation of the aircraft arise mainly from the control of noise and emissions. For civil aircraft such regulations are vested in separate operational regulations.3 For light aircraft, some airfields have locally applied operation restrictions to avoid noise complaints from adjacent communities. Such issues are becoming increasingly significant to aircraft design. “chap02” — 2003/3/10 — page 13 — #8 13
  34. 34. 14 Aircraft Design Projects 2.3.4 Commercial and manufacturing considerations Political issues may affect the way in which the aircraft is to be manufactured. Large aircraft projects will involve a consortium of companies and governments (e.g. Airbus). This will directly dictate the location of design and manufacturing activity. Such influence may also extend to the supply of specific systems, engines and components to be used on the aircraft. If such restrictions are to be applied, the design team should be aware of them as early as possible in the design process. 2.3.5 Systems and equipment requirements Aircraft manufacture is no longer just concerned with the supply of a suitable airframe. All aircraft/engine and other operational systems have a significant influence in the overall design philosophy. Today many aircraft are not technically viable without their associated flying and control systems. Where such integration is to be adopted the design team must include this in the aircraft requirements. This aspect is particularly significant for the design of military aircraft that rely on weapon and other sensor systems to function effectively (e.g. stealth). Regulations for military aircraft usually fully describe the systems that the airframe must support. 2.4 Configuration options With a fully described set of regulations, knowledge of existing aircraft data and a complete understanding of the problem, it is now possible to start the technical design tasks. Many project designers regard this stage as the best part of all the design processes. The question to be answered is simply this: Starting with a completely clear mind, what configurational options can you suggest that may solve the problem? For example, a two-seat light touring aircraft could be: side-by-side or tandem seating, high or low wing, tractor or pusher engine, canard or tail stabilised, nose or tail wheeled, conventional or novel planform (e.g. box wing, joined wing, delta, tandem), etc. The following stage of the design process will sort through the ‘weird and wonderful’ configurations to eliminate the unfeasible and uncompetitive layouts. At this point in the layout process a quantity of ideas is needed and a judgement on their suitability will be left until later. With this in mind it is unnecessary to elaborate on an option past the point at which its characteristics can be appreciated. A good starting point for this work is to list the configurations that past and existing aircraft of this type have adopted. A brief synopsis of the strength and weaknesses of each option may be written so that improvements to the designs can be identified. Such analysis will also help in the concept-filtering phase that will follow. In the conceptual design stage, designers have two options available for their choice of engines. Namely a ‘fixed’ (i.e. a specified/existing or manufacturers’ projected engine), or an ‘open’ design (in which the engine parameters are not known). In most cases, and definitely at later stages in the design process, the size and type of engine will have been determined. The aircraft manufacturer will prefer that more than one engine supplier is available for his project. In this way he can be more competitive on price and supply deadlines. For design studies in which the engine choice is open, it is possible to adopt what is known as a ‘rubber’ engine. Obviously, such engines do not exist in practice. The type and initial size of the rubber engine can be based on existing or “chap02” — 2003/3/10 — page 14 — #9
  35. 35. Preliminary design Number of seats (3 class) 380 340 300 260 4000 5000 6000 7000 8000 9000 Aircraft range (with reserves) (nm) Fig. 2.2 Aircraft development programme (Boeing 777) engine manufacturers’ projected engine designs. Using a rubber engine, the aircraft designer has the opportunity to scale the engine to exactly match the optimum size for his airframe. Such optimisations enable the designer to identify the best combination of airframe and engine parameters. If an engine of the preferred size is not available, in the timescale of the project, the designer will need to reconfigure the airframe to match an available engine. Rubber engine studies show the best combination of airframe and engine parameters for a design specification and can be used to assess the penalties of selecting an available engine. Aircraft and engine configuration and size is often compromised at the initial design stage to allow for aircraft growth (either by accidental weight growth or by intent (aircraft stretch)). Such issues must be kept in mind when considering the various options. Most aircraft projects start with a single operational purpose but over a period of time develop into a family of aircraft. Figure 2.2 shows the development originally envisaged by Boeing for their B777 airliner family. For military aircraft such developments are referred to as multi-role (e.g. trainer, ground support, etc.). It is important that designers appreciate future developments at an early design stage and allow for such flexibility, if desired. 2.5 Initial baseline sizing At the start of this stage you will have a lot of design options available together with a full and detailed knowledge of the problem. It would be impossible and wasteful to start designing all of these options so the first task is to systematically reduce the number. First, all the obviously unfeasible and crazy ideas should be discarded but be careful that potentially good ideas are not thrown out with the rubbish. Statements and comments in the aircraft regulations and the problem definition reports will help to filter out uneconomic, weak and ineffective options. The object should be to reduce “chap02” — 2003/3/10 — page 15 — #10 15
  36. 36. 16 Aircraft Design Projects the list to a single preferred option but sometimes this is not possible and you may need to take another one or two into the next design stage. Obviously, the workload will be increased in the next stages if more options are continued. Eventually it will be necessary to choose a single aircraft configuration. This will mean that all the work on the rejected options may be wasted. This can be a very difficult part of the design process for a student design team. At this point, it is common for each member of the team to have invested a lot of time and energy into his or her own proposed design concept. It is often difficult to get team members to release their emotional ties to their own proposals and begin to embrace those of others or even to find a viable compromise. To get through this stage of the process both good team management and an effective means of comparing and evaluating all proposed concepts are required. Some of these difficulties are discussed in Chapter 11 (section 11.2). All proposed solutions to the design objective need to be given a fair and impartial assessment during the selection of the final concept. Obviously, a compromise solution which draws upon key elements of every team member’s contributions will result in a happier set of team players. On the other hand, it is important that the selected concept embodies the best design elements that the team has developed. These must be chosen for the benefit of the overall design and not just to keep each member of the team happy. Once decisions have been made on the configuration(s) to be further considered it is necessary to size the aircraft. A three-view general arrangement scale drawing for each aircraft configuration will be required. Little detail will be known at this stage about the aircraft parameters (wing size, engine thrust, and aircraft weight) so some crude estimates have to be made. This is where data from previous/existing aircraft designs will be useful. Although the new design will be different from previous aircraft, such inconsistencies can be ignored at this stage. Use representative values from one or a small group of the specimen aircraft for wing loading, thrust loading and aircraft take-off weight. It is also possible to use a representative wing shape and associated tail sizes. The design method that follows is an iterative process that usually converges on a feasible configuration quickly. The initial general arrangement drawing, produced to match existing aircraft parameters, provides the starting point for this process. Even though your design is relatively crude at this stage it is important to draw it to scale making approximations for the relative longitudinal position of the wing and fuselage and the location of tail surfaces and landing gear. Most aircraft layouts start with the drawing of the fuselage. For many designs the geometry of the fuselage can be easily proportioned as it houses the payload and cockpit/flight deck. These parameters are normally specified in the project brief. They can be sized using design data from other aircraft. The non-fuselage components (e.g. wing, tail, engines and landing gear) are added as appropriate. With a reasonable first guess at the aircraft configuration, the aircraft can be sized by making an initial estimate of the aircraft mass. Once this is completed it is possible to more accurately define the aircraft shape by using the predicted mass to fix the wing area and engine size. 2.5.1 Initial mass (weight) estimation The first step is to make a more accurate prediction of the aircraft maximum (take-off ) mass/weight. (Note: if SI units are used for all calculations it is appropriate to consider aircraft mass (kilograms) in place of aircraft weight (Newtons).) “chap02” — 2003/3/10 — page 16 — #11
  37. 37. Preliminary design Aircraft design textbooks4,5,6 show that the aircraft take-off mass can be found from: MTO = where MTO = MUL∗ = ME∗ = MF = MUL 1 − (ME /MTO ) − (MF /MTO ) maximum take-off mass mass of useful load (i.e. payload, crew and operational items) empty mass fuel mass (*When using the above equation it is important not to double account for mass components. If aircraft operational mass is used for ME , the crew and operational items in MUL would not be included. One of the main difficulties in the analysis at this stage is the variability of definitions used for mass components in published data on existing aircraft. Some manufacturers will regard the crew as part of the useful load but others will include none or just the minimum flight crew in their definition of empty/operational mass. Such difficulties will be only transitional in the development of your design, as the next stage requires a more detailed breakdown of the mass items.) The three unknowns on the right-hand side of the equation can be considered separately: (a) Useful load The mass components that contribute to MUL are usually specified in the project brief and aircraft requirement reports/statements. (b) Empty mass ratio The aircraft empty mass ratio (ME /MTO ) will vary for different types of aircraft and for different operational profiles. All that can be done to predict this value at the initial sizing stage is to assume a value that is typical of the aircraft and type of operation under consideration. The data from existing/competitor aircraft collected earlier is a good source for making this prediction. Figure 2.3 shows how the data might be viewed. Alternatively, aircraft design textbooks often quote representative values for the ratio for various aircraft types. Empty mass (ME) Two engines Three engines Four engines Slope (ME /M TO) Two engines = 0.55 More than two = 0.47 Max. take-off mass (MTO) Fig. 2.3 Analysis of existing aircraft data (example) “chap02” — 2003/3/10 — page 17 — #12 17
  38. 38. 18 Aircraft Design Projects a – take-off, b – climb, c – cruise, d – step climp, e – continued cruise, f – descent, g – diversion, h – hold, i – landing at alternate airstrip. c d e f g b h a i Fig. 2.4 Mission profile (civil aircraft example) (c) Fuel fraction For most aircraft the fuel fraction (MF /MTO ) can be crudely estimated from the modified Brequet range equation: MF 1 · (time) = (SFC) · (L/D) MTO where (SFC) = engine specific fuel consumption (kg/N/hr) (L/D) = aircraft lift to drag ratio (time) = hours at the above conditions The mission profile will have been specified in the project brief. Figure 2.4 illustrates a hypothetical profile for a civil aircraft. This shows how the mission profile consists of several different segments (climb, cruise, etc.). The fuel fraction for each segment must be determined and then summed. Reserve fuel is added to account for parts of the mission not calculated. For example: (a) for the fuel used in the warm-up and taxi manoeuvres, (b) for the effects on fuel use of non-standard atmospheric conditions (e.g. winds), (c) for the possibility of having to divert and hold at alternative airfield when landing. The last item above is particularly significant for civil operations. In such applications designers sometimes convert the actual range flown to an equivalent still air range (ESAR) using a multiplying factor that accounts for all of the extra (to cruise) fuel. When using the Brequet range equation it must be remembered that both engine (SFC) and aircraft (L/D) will be different for different flight conditions. These variations arise because the aircraft speed, altitude, weight and engine setting will be different for each flight segment. Typical values for (SFC) can be found in engine data books7 or from aircraft and engine textbooks4,8 for the type of engine to be used. The aircraft lift to drag ratio (L/D) will vary and be dependent on aircraft geometry (particularly wing angle of attack). Such values are not easily available for the aircraft in the initial design stage. However, we know that previous designers have tried to achieve a high value in the principal flight phase (e.g. cruise). We can use the fact that in cruise “chap02” — 2003/3/10 — page 18 — #13
  39. 39. Preliminary design ‘lift equals weight’ and ‘drag equals thrust’. We can therefore transpose (L/D) into (W /T ). Both aircraft weight and engine thrust (at cruise) could be estimated from our specimen aircraft data. This value will be close to the maximum (L/D) and relate only to the cruise condition. At flight conditions away from this point the value of (L/D) will reduce. It must be stressed that the engine thrust level in cruise will be substantially less than the take-off condition due to reduced engine thrust setting and the effect of altitude and speed. This reduction in thrust is referred to as ‘lapse rate’. Engine specific fuel consumption will also change with height and speed. Values for (L/D) vary over a wide range depending on the aircraft type and configuration. Typical values range from 30 to 50 for gliders, 15 to 20 for transport/civil aircraft, 12 to 15 for smaller aircraft with reasonable aspect ratio and less than 10 for military aircraft with short span delta wing planforms. Aircraft design textbooks are a source of information on aircraft (L/D) if the values cannot be estimated from the engine cruise conditions and aircraft weight. (Time) is usually easy to specify as each of the mission segments is set out in the project brief (mission profiles). Alternatively, it can be found by dividing the distance flown in a segment by the average speed in that segment. 2.5.2 Initial layout drawing Obviously, all the above calculations require a lot of ‘guesstimation’ but at least at the end we will have a better estimate of the aircraft maximum take-off mass than previously. This value can then be used in conjunction with the previously assumed values for wing and thrust loading to refine the size of the wing and engine(s). The original concept drawing can be modified to match these changes. This drawing becomes the initial ‘baseline’ aircraft configuration. 2.6 Baseline evaluation The methods used up to this point to produce the baseline aircraft configuration have been based mainly on data from existing aircraft and engines. In the next stage of the design process it is necessary to conduct a more in-depth and aircraft focused analysis. This will start with a detailed estimation of aircraft mass. This is followed by detailed aerodynamic and propulsion estimates. With aircraft mass, aerodynamic and engine parameters better defined it is then possible to conduct more accurate performance estimations. The baseline evaluation stage ends with a report that defines a modified baseline layout to match the new data. A brief description of each analysis conducted in this evaluation stage is given below. 2.6.1 Mass statement Since the geometrical shape of each part of the aircraft is now specified, it is possible to make initial estimates for the mass of each component. This may be done by using empirical equations, as quoted in many design textbooks, or simply by assuming a value for the component as a proportion of the aircraft maximum or empty mass. Such ratios are also to be found in design textbooks or could match values for similar aircraft types, if known. The list below is typical of the detail that can be achieved. “chap02” — 2003/3/10 — page 19 — #14 19
  40. 40. 20 Aircraft Design Projects Generating a mass statement like this one is the first task in the baseline evaluation phase. Wing (MW ) Tail (MT ) Body (MB ) Nacelle (MN ) Landing gear (MU ) Control surfaces (MCS ) total aircraft structure (MST ) Engine basic (dry) Engine systems Induction (intakes) Nozzle (exhaust) Installation total propulsion system (M P ) Aircraft systems and equipment (MSE ) aircraft empty mass = M E = M ST + M P + M SE Operational items (MOP ) aircraft operational empty mass (M OE ) = M E + M OP Crew* (MC ) Payload (MPL ) Fuel (MF ) aircraft take-off mass (M TO ) = M OE + M C + M PL + M F (*For some military aircraft mass statements, the crew are considered to form part of the operational items and their mass is added to aircraft OEM.) The main structural items in the list above (e.g. wing, fuselage, engine, etc.) can be estimated using statistically determined formulae which can be found in most aircraft design textbooks. (Note: if you are working in SI units be careful to convert mass values from historical reports, journals, and current US textbooks to kilograms (1 kg = 2.205 lb).) Many of these mass items are dependent on MTO , therefore estimations involve an iterative process that starts with the assumed value of MTO , as estimated in the initial sizing stage. Spreadsheet ‘solver’ methods will be useful when performing this analysis. At the early design stages, the estimation of mass for some of the less significant (and smaller) components may be too time consuming to calculate in detail (e.g. tail, landing gear, flight controls, engine systems and components, etc.). To speed up the evaluation process, these can be estimated by assuming typical percentage values of MTO , as mentioned above. Such values can be found from existing aircraft mass breakdowns, if available, or from aircraft design textbooks. At the final stages of the conceptual phase an aircraft mass will be selected which is slightly higher than the estimated value of MTO . This higher weight is known as the ‘aircraft design mass’. All the structural and system components will be evaluated using the value for the aircraft design weight as this provides an insurance against weight growth in subsequent stages of the design process. For aircraft performance estimation, the mass to be used may be either the MTO value shown above or something less (e.g. military aircraft manoeuvring calculations are frequently associated “chap02” — 2003/3/10 — page 20 — #15
  41. 41. Preliminary design with the aircraft operational empty mass plus defensive weapons and half fuel load only). 2.6.2 Aircraft balance With the mass of each component estimated and with a scale layout drawing of the aircraft it is possible, using educated guesses, to position the centre of mass for each component. This will allow the centre of gravity of the aircraft in various load conditions (i.e. different combinations of fuel or payload) to be determined. It is common practice to estimate the extreme positions (forward and aft) so that the trim loads on the control surfaces (tail/canard) and the reaction loads on the undercarriage wheels can be assessed. Up to this point in the design process, the longitudinal position of the wing along the fuselage has been guessed. As part of the determination of the aircraft centre(s) of gravity, it is possible to check this position and, by iteration, to reposition it to suit the aircraft lift and inertia force (i.e. mass × acceleration) vectors. This process is referred to as ‘aircraft balancing’. As moving the wing will affect the position of the aircraft centre of gravity and the wing lift aerodynamic centre from the datum, several iterations may be required. There are several methods that can be used to reduce the complications inherent in this iteration. The simplest method sets the position of the aircraft operational empty mass relative to a chosen point (per cent chord aft of the wing leading edge) on the wing mean aerodynamic chord line. To start the process the aircraft operational empty mass components are divided into two separate groups: (a) Wing mass group (MWG ) (and associated components) – this will include the wing structure, fuel system (if the fuel is housed in the wing), main landing gear unit (even if it is structurally attached to the fuselage), wing mounted engines and all wing attached systems. (b) Fuselage mass group (MFG ) (and associated components) – this group will include the fuselage structure, equipment, cockpit and cabin furnishings and systems, operational items, airframe services, crew, tail structure, nose landing gear and fuselage mounted engines and systems. Note: if the position of wing mounted engines is linked to internal fuselage layout requirements (e.g. propeller plane be in line with non-passenger areas) then these masses should be transferred to the fuselage group. Obviously all the aircraft components relating to the aircraft operational empty mass must be included in either of the above groups (i.e. MOE = MWG +MFG ). It is important to check that none of the component masses has been omitted before starting the balancing process. It is possible to determine the centres of mass separately for each of the two mass groups above. The distance of the wing group centre of mass from the leading edge of the wing mean aerodynamic chord (MAC) is defined as XWG (see Figure 2.5a). The next stage is to select a suitable location for the centre of gravity of the aircraft operational empty weight, on the wing mean aerodynamic chord. If the centre of gravity is too far aft or forward then the balancing loads from the tail (or canard) will be high. This will result in a requirement for larger tail surfaces and thereby increased aircraft mass and trim drag. For most conventional aircraft configurations, a centre of gravity position coincident with the 25 per cent MAC position behind the wing leading edge is considered a good starting position. If it is known that loading the aircraft from the operational empty mass will progressively move the aircraft centre of gravity forward, “chap02” — 2003/3/10 — page 21 — #16 21
  42. 42. 22 Aircraft Design Projects then a 35 per cent MAC position would be a better starting point. Such cases arise on civil airliners with rear fuselage mounted engines. Conversely, a 20 per cent MAC would be chosen for designs with mainly aft centre of gravity movements. For aircraft flying at supersonic speed the centre of lift will be at about the 50 per cent MAC position. This must be carefully allowed for when selecting the operational mass position. The location of the chosen operational empty mass location with respect to the leading edge of the wing mean aerodynamic chord is defined as XOE . It is possible to take moments of the aircraft masses shown in Figure 2.5a. By rearranging the moment equation, the position of the fuselage group mass relative to line XX can be calculated. The resulting equation is shown below: XFG = XOE + (XOE − XWG )(MWG /MFG ) Overlays of the separate wing and fuselage layouts provide the best method of fixing the wing relative to fuselage. On a plan view of the wing, determine the position of the wing MAC and its intersection with the wing leading edge (line XX). Also, on this drawing show the position of the wing group centre of mass, see Figure 2.5b. Measure the distance XWG from this drawing and use it in the formula above together with the selected value of XOE and the calculated wing and fuselage group masses (MWG and MFG ), to evaluate the distance XFG . On a plan view of the fuselage, determine the position of the fuselage group mass centre (using any convenient datum plane) then draw a line XX at a distance XFG forward of this position, as shown in Figure 2.5c. Overlay the wing and fuselage diagram lines XX. This is the correct location of wing and fuselage to give the aircraft operational centre of gravity at the previously selected position on the wing MAC. It is not unusual to discover by this process that the originally assumed position of the wing relative to the fuselage, on the aircraft layout drawing, is incorrect and must be changed. With the aircraft balanced, it is now possible to determine the range of aircraft centre of gravity movement about the operational empty position and to assess the effect of this on the tail sizing. Obviously, it is preferable to design for small movements of the aircraft centre of gravity to ensure the control forces are small. To do this, the disposable items of mass (fuel and payload) should be centred close to the aircraft operational empty centre of gravity position as practical. At this stage in the development of the aircraft geometry it is possible to position the undercarriage units. The process involves geometric and load calculations associated with the aircraft mass and centre of gravity range. The main units must allow for adequate rotation of the aircraft on take-off and in the landing attitude. When the aircraft is in the maximum tail down attitude, the aircraft rearmost centre of gravity position must be forward of the wheel reactions. This will ensure the aircraft does not stay in this position. The loads on the main and nose units can be determined by simple mechanics. Make sure that the nose wheel load is not excessive as this will require a large tailplane force to lift the nose on take-off. On the other hand, if the load is too small on the nose wheel it will not generate an effective steering force. The forces determined for each unit will dictate the tyre size commensurate with the allowable tyre pressure and runway point-load capability. Several aircraft design textbooks include undercarriage layout guidelines. 2.6.3 Aerodynamic analysis At the same time as the mass and balance estimation is made, or sequentially after if you are working alone, it is possible to make the initial estimations for the baseline aircraft aerodynamic characteristics (drag and lift). The aircraft drag estimation, like mass, “chap02” — 2003/3/10 — page 22 — #17
  43. 43. Preliminary design (a) X XFG MFG MOE = L XOE Required position of aircraft MOE centre of gravity X WG M WG X CT (b) CT C/2 Chord line X Intersection of wing MAC with LE Wing MAC CT XOE Position of M WG centre of gravity Assumed position of aircraft MOE behind MAC leading edge X (c) X X Position of MFG centre of gravity XFG As calculated from formula Fig. 2.5 Aircraft balance methodology (diagrams a, b and c) “chap02” — 2003/3/10 — page 23 — #18 23